Have a 'file' generate its content on demand?

Question

Is there a way that I can have something which appears as a file, but when being read some code executes which fills the content of the file "on the fly"?

Like FIFO/ named pipe, but without the need to start the writing process by hand.

Or like CGI on webservers, but on the local file system.

EDIT:
To guard myself against "why I want this"-doubts:
I want this to be able to have an audiofile whose content is generated on access, and then delivered to the audio player. And it should not be streaming, since when accessed by the audio player the length of the audio should be known.

A named pipe is the way to go. If you don't want to start the writing process by hand, turn it into a daemon and start it at boot. — Henrik supports the community, Apr 02 '16 at 10:40
@ohno: but I want to be able to specify an arpitrary command/ script to generate the content. — Golar Ramblar, Apr 02 '16 at 13:51
Yes, http://unix.stackexchange.com/questions/66990/create-a-virtual-file-that-is-actually-a-command answers this question for me (scriptfs, or maybe fuseflt can also do the job). — Golar Ramblar, Apr 03 '16 at 06:01

score 12 · Answer 1 · answered Apr 02 '16 at 10:59

12

If you are willing to do some coding, the fanotify system call allows you write a program that will intercept reads on any file beneath some mount point, and substitute or manipulate the data before returning it. See man fanotify for example C code.

Alternatively, you can mount a fuse filesystem, see man fuse, which allows you to implement a filesystem by a userspace program. For example, curlftpfs (see its man page) converts file accesses into curl ftp calls.

answered Apr 02 '16 at 10:59

meuh

51,383

Is the FA in fanotify for "File Access?" – Aaron Hall Apr 02 '16 at 12:33
The FA might be for "file activities", as this phrase is used in the man page, though it is not explicitly defined. – meuh Apr 02 '16 at 12:39

Have a 'file' generate its content on demand?

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